Problem: $ { {-3} \times \left[\begin{array}{r}{2} \\ {1} \\ {3}\end{array}\right] = {?} } $
Answer: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{-3}\times{2} \\ {-3}\times{1} \\ {-3}\times{3}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{-6} \\ {-3} \\ {-9}\end{array}\right]}$